As shown in FIG. 1, a network of seismic streamers 10, referenced 10a to 10e, is towed by a seismic vessel 11. A seismic streamer 10 generally includes a seismic telemetry cable, extending along the seismic streamer 10 and adapted to transmit seismic data towards the vessel.
A seismic streamer 10 generally includes different kinds of seismic devices, i.e. devices involved in the management of the seismic data, and especially:                seismic sensors 30, such as hydrophones or geophones or accelerometers or the like, arranged along the streamer 10 and adapted for detecting acoustic signals;        electronic units, also called nodes 20, distributed in series along the streamer 10 at intervals that are not necessarily regular, each node 20 being associated with a given set of seismic sensors 30, the nodes 20 particularly processing seismic data issued from seismic sensors 30;        telemetry modules 40, also called concentrators, arranged along the seismic cable of the streamer 10, each associated with a given set of nodes 20 in particular for providing power supply and retrieving seismic data acquired by the nodes.        
The seismic telemetry cable may also be adapted to transmit seismic quality control data, also called “QC data”, towards or from the seismic vessel 11. These data relate to the quality control of the aforementioned seismic devices during their functioning, such as battery level, sensor status, memory status, synchronization availability, etc. For the sake of simplification, we shall refer in the following description to seismic data. It should be noted that seismic quality control data are applicable as well in this description.
The seismic streamer 10 also includes auxiliary equipment, i.e. “non-seismic devices” or “devices not involved in the management of the seismic data”. Such auxiliary equipment may be:                a head buoy 12 which typically supports the head end of the streamer 10 connected to the vessel 11;        a tail buoy 13 which typically supports the tail end of the streamer 10;        navigation control devices, commonly referred to as positioning birds 14, which are installed at intervals that are not necessarily regular, for example every 50, 150, 300 or 450 meters, along the streamer 10 and used to control the depth and lateral position of the streamer 10;        environmental sensors, not shown in the figures;        cameras, not shown in the figures;        etc.        
In other words, auxiliary equipment supplements the aforesaid seismic devices such as the seismic sensors, nodes and telemetry modules, to allow a good operation of the seismic acquisition system.
FIG. 2 illustrates in detail the block referenced A in FIG. 1, which is a portion of the streamer 10a. In FIG. 2, each bird 14 includes a body 1 equipped with motorized pivoting wings 2 allowing to modify the position of the streamers laterally between them by a horizontal driving and drive the streamers in immersion by a vertical driving. The nodes 20 are represented by hatched squares in FIGS. 1 and 2.
The nodes 20 are connected to the concentrators 40 via electrical wires, not shown in the figures. More precisely, all the nodes 20 are arranged in series along the electrical wires from the head end to the tail end of the streamer 10. Each node 20 is associated with a given set of seismic sensors 30 and is adapted in particular to collect seismic data issued from this set of seismic sensors 30 and to digitize them, if necessary, before sending them, via the concentrators 40 and the seismic telemetry cable, towards the vessel 11. Control data may be also transmitted from the vessel towards the nodes 20, via the concentrators 40 and the seismic telemetry cable, for proper functioning of the seismic acquisition system.
Concentrators 40 are assembled in series along the streamer 10. Each concentrator 40 is associated with one or several nodes 20 for providing electrical power supply to these nodes 20 and for concentrating the seismic data issued from these nodes 20. Then, the concentrators 40 transmit the concentrated data towards the vessel 11 via the seismic telemetry cable in order to be processed in the central unit located onboard the vessel 11.
A known node assembly includes a single node with a horseshoe-shaped electronic unit casing and a complementary part for forming a generally cylindrical shape. The casing and complementary part are made of a metal, especially aluminum. In the horseshoe-shaped casing, there is a hollow horseshoe-shaped space for housing electronics, especially an electronic board. A disadvantage of such node is that the added weight to the streamer is high. The added weight alters the overall streamer weight, which is of prime importance in the seismic industry. The streamer needs particular buoyancy so that the streamer remains flat at a particular depth. Also, in such node, the electronic unit casing is prevented from water ingress thanks to a potting resin which is filled in the electronic board housing, sealing the node. However, the use of such potting resin makes the node manufacturing process very complex in terms of replicability and generates severe node design constraints, in particular inducing stresses on the electronic components when submitted to temperature change, the differential thermal coefficient of expansion between the different materials being high.
Another known assembly includes a cylindrical casing made of a metal material, especially aluminium, said casing being split into two symmetrical half cylindrical sub casing. Said casing forms a cylinder having a central hollow part for housing a portion of cable of the streamer and a half-cylindrical hollow space placed around the hollow core in each subcasing for housing electronic boards. Said casing houses two node electronics at one single location, which double the risk of node failure due to a particular damage at this specific location. Furthermore, said casing, including two node electronics, limits the added weight to the streamer but has the disadvantage to be longer than the previous embodiment. Indeed, the streamer with the assembly is to be winded up on a reel when stored. The longer the assembly, the greater the bending forces with which the electronic unit has to cope. Limiting the assembly length minimizes the overall forces and decreases consequently the risk of mechanical failure. Such an assembly also requires the addition of supplemental mounting pieces positioned at lateral ends of the assembly for mounting the nodes onto the streamer. These supplemental pieces add supplemental length to the assembly, so that the available length for the electronics is to be reduced for a similar size. The space for electronics is therefore not optimized.